Gently mix the water, washing-up liquid and glycerin inside the
washing up bowl to make the bubble mixture.

Stretch the coat hangers into a circular shape. These are your
rackets.

Dip the rackets in the bubble mix and remove slowly, so that
the bubble mixture fills the circle.

To make your tennis ball, dip one end of the pen tube into the
bubble mix. Blow through the other end to make a bubble.

Play bubble tennis!

Be gentle with the tennis ball and rackets, otherwise you'll end
up with bubble mix everywhere! How many times can you hit the ball
before it bursts? Does it matter how big the ball is? Does adding
more or less glycerin make a difference?

Here's a video of some people playing with strong bubbles in
their chemistry class.

What's happening?

Bubble mixture is a detergent. Detergents are made of molecules
that have a head and a tail. The head of the molecule wants to be
near water. The tail of the molecule wants to be as far away from
water as possible.

When your bubble hits the bubble racket, they repel each other.
The tails on the bubble want to stay away from the water in the
bubble racket and vice versa. This means the bubble and racket
don't stick to each other.

Bubbles burst when the layer of water and detergent get too
thin. The pressure of the air inside the bubble causes the bubble
to pop. Glycerin makes the bubbles last longer because it helps to
stop the layer of water getting too thin.